Conventionally, for example, in the field of optical technology to which liquid crystal display devices and the like belong, it has been suggested to use a light diffusion film which can diffuse an incident light coming from a particular direction into particular directions, while transmitting straight an incident light coming from any other directions.
A variety of forms of such a light diffusion film are known; however, in particular, light diffusion films having, within the films, a louver structure in which plural plate-shaped regions having different refractive indices are alternately arranged in one arbitrary direction along the film plane, have been widely used (for example, Patent Document 1).
Namely, Patent Document 1 discloses a method for manufacturing a light control plate (light diffusion film) which includes a first step of retaining on a film a resin composition composed of a plurality of compounds, each having one or more polymerizable carbon-carbon double bonds in their molecules and having a refractive index that is different from the refractive indices of the other compounds, irradiating the composition with ultraviolet radiation from a specific direction and thereby curing the resin composition; and a second step of retaining the resin composition on the film of the cured product thus obtained, irradiating with ultraviolet radiation from a direction different from that of the first step, and thereby curing the resin composition, characterized in that the second step is repeated if necessary.
On the other hand, regarding another type of light diffusion films, light diffusion films having, within the film, a columnar structure in which plural pillar-shaped objects having a relatively high refractive index are arranged to stand close together in a region having a relatively low refractive index, have been widely used (for example, Patent Documents 2 and 3).
Namely, Patent Document 2 discloses a manufacturing device by which a light control plate (light diffusion film) is formed by opposing a linear light source to a photocurable resin composition film, and, while either the photocurable resin composition film or the linear light source, or both are being moved, by irradiating the photocurable resin composition film with light from the linear light source, thereby curing the composition. The device for manufacturing a light control plate (light diffusion film) is a device in which the axial direction of the linear light source crosses the moving direction, and in which a plurality of light blocking thin plates opposed to one another are provided between the photocurable resin and the linear light source at a predetermined interval in a direction almost perpendicular to the moving direction, in such a fashion that one edge facing the photocurable resin composition of each of the light blocking thin plates is parallel to the moving direction.
Furthermore, Patent Document 3 discloses a method for manufacturing an anisotropic diffusion medium (light diffusion film), the method including providing a composition containing a photocurable compound into a sheet form, irradiating this sheet with parallel light rays from a predetermined direction P to cure the composition, and thereby forming an aggregate of plural rod-shaped cured regions that are extended in parallel to the direction P, within the sheet, characterized in that aggregates of tubular objects that are arranged in parallel to the direction P are interposed between a linear light source and the sheet, and light irradiation is performed through these tubular objects.
Furthermore, there has been suggested a light diffusion film having both the louver structure and the columnar structure described above, within the film (for example, Patent Document 4).
That is, Patent Document 4 discloses a light diffusion film having a first structural region for anisotropically diffusing incident light, and a second structural region for isotropically diffusing incident light, characterized in that the first structural region is a louver structure region in which plural plate-shaped regions having different refractive indices are arranged alternately in parallel along the direction of the film plane, and the second structural region is a columnar structure region in which plural pillar-shaped objects are arranged to stand close together in a medium, with the plural pillar-shaped objects having a refractive index different from that of the medium.